Hi angel.
When I first started out I was with an agency.
At my interview I just dressed in my normal clothes, ie leggings and a nice top !

For out calls I wore leggings (I literally live in leggings  ) and a smart top, like a blouse or something from the 'career' section in shops. I wore flat shoes cause I was convinced I looked suspicious in my heels, so I always had heels in my bag!
For in calls, I done them from the agencies apartment, so I just wore my underwear and heels with a short satin robe over it! Or sometimes I'd wear a nice dress..wear what you feel comfortable in!! No point in wearing something overly sexy, if you aren't confident in wearing it 
Good luck honey xx

I had an interview and the owner said "come as you would for an outcall" so I guess a dress is a must and hair/makeup top notch. This was ?300+ agency though

I think it depends on the type of agency, there are quite a few different markets out there...I joined a well know London agency and they expected me to look like a hooker when visiting hotels ! I was told that I was too conservative and they wanted me to wear very short skirts, false eyelashes and lots of heavy makeup etc..also I had to come into London by train so this was not practical.

I quickly left the agency and had an interview with another one, they loved my more conservative look as that's what their clients wanted and told me that theirs was a totally different market..it was really interesting